PROJECT SUMMARY Background and Objectives: Synapses and circuits possess a robust capacity for stress response. They employ homeostatic regulatory mechanisms to maintain physiologically appropriate levels of synaptic output. Improved knowledge about homeostatic forms of synaptic plasticity should lead to a better understanding of neurological disorders that occur when synapse stability is lost. Using genetic and electrophysiological approaches at the model Drosophila neuromuscular junction (NMJ) synapse, three new factors required for the long-term homeostatic maintenance of NMJ function were uncov- ered: two tyrosine kinase signaling molecules residing in the muscle and one phospholipase C-? (PLC?) molecule residing in the neuron. The objective of this proposal is to understand how these three molecules integrate cell-cell signaling processes to maintain synapse stability throughout life. Specific Aims and Research Design: This project has three specific aims. The first two aims will delineate how each respective tyrosine kinase drives a muscle-to-nerve signaling process to stabilize synaptic activity over long periods of developmental time. The third aim will address how neuronal PLC? integrates cell-cell signals at the synapse to autonomously control neuronal output. Each aim will combine electrophysiology, genetics, pharmacology, biochemistry, and synapse imaging. A prima- ry assay for each aim will be to challenge NMJ function ? usually by inhibiting glutamate receptors in the muscle ? and then to examine the NMJ by electrophysiology to check if it appropriately responds to that challenge by releasing more glutamate from the neuron. By combining this electrophysiological approach with synapse imaging it will be possible to identify manipulations that specifically impair synapse function ? as opposed to other parameters, like synapse growth. The expected outcome is a detailed model of how synaptic tissues transmit cell-cell signals to maintain stable activity levels. Health Relatedness: Neurological disorders like epilepsy, ataxia, and migraine are associated with unstable neuronal function. Therefore, understanding how synapses work to maintain stability on a molecular level could have profound implications for disorders with underlying neuronal instabilities. Yet the cell-cell signaling events that tightly control levels of synaptic output are poorly understood. The genetically tractable Drosophila NMJ employs homoestatic strategies to stabilize synapse function ? such as altering levels of presynaptic calcium influx ? that are shared by mammalian central synapses. Taking advantage of the molecular and genetic tools offered by the NMJ promises to shed light on universally conserved mechanisms of how synapses maintain stable function throughout life.